Luxury Goods Group, an Italian company that sells sunglasses, reported net sales of $9\mathrm{.}4$ billion in $2018$ and $10\mathrm{.}2$ billion in $2017.$ Gross profit decreased from $4\mathrm{.}5$ billion in $2017$ to $3\mathrm{.}8$ billion in $2018.$

Calculate the gross profit percentage for each year. $($Round your answers to $1$ decimal place.$)$

The change in gross profit was caused by: